An ocularum (Plural: oculara) is a magical object that appears as a skull with a crystal placed in the right eye socket through the back of the skull.
They are found throughout the world, usually placed at high elevations upon pedestals. Their purpose seems to be to reveal the locations of magical shards that were previously hidden until the Breach somehow disrupted the magic that had been hiding them from view. Looking through the crystal in the skull reveals the glow of the shards, pinpointing their location.
Finding the first shard, regardless of which ocularum you use, will trigger the war table operation Investigate the Shards. Enchanter Renaud finds a scroll that makes mention of the shards, and after doing some research discovers that they may be able to unlock an elven temple in the Forbidden Oasis called Solasan. Collecting the shards enables the completion of several side quests (see Collections for more details), and once all shards have been collected the final door in the very heart of the temple can be opened.
See also EditCodex entry: Ocularum Codex entry: Oculara Codex entry: A Worn Diary Codex entry: Leather-Bound Hakkonite Journal